The present invention relates to communication systems. More particularly, and not by way of limitation, the present invention is directed to a method and apparatus for reducing Push-to-Talk (PTT) latency in a Wideband Code Division Multiple Access (WCDMA) radio communication network.
In the PTT solution for the IP Multimedia Subsystem (IMS) Key Performance Indicator (KPI) values are used to demonstrate the expected end-to-end latency that users will experience when utilizing the PTT service. For many users, it is undesirable to utilize WCDMA for PTT service because large latencies associated with WCDMA provide a poor PTT experience for the user. Other solutions have been developed for reducing KPI values in GSM (GPRS/EDGE) for the terminating user. These solutions, such as the one disclosed in WO 2004/102997 however, looked only at reducing delays on the terminating side because in GPRS/EDGE, paging is the only real delay in the network.
Currently when an originating user in a WCDMA network presses the PTT button on his terminal, i.e., User Equipment (UE) indicating he is ready to initiate a PTT session, an application in the UE builds an INVITE message and sends the message on the WCDMA network to a Call/Session Control Function (CSCF). At this point, the UE sends a “Connection Request” message (if the UE is in IDLE state) or a “Cell Change” message (if the UE is in URA_PCH state) to obtain a Radio Bearer (RB) in the WCDMA network with which to send the INVITE message.
FIG. 1 is a call flow diagram illustrating the flow of messages in an existing procedure for initiating a PTT session in a WCDMA network when a UE 10 is in URA_PCH state. At step 11, the user pushes a PTT button on the UE, and the UE sends a Cell Update message 12 on the Random Access Channel (RACH) to the Radio Network Controller (RNC) 13. It requires about 200-250 ms to send the Cell Update message because the UE must ramp up its power to send the message, and the UE must read SIB 7 before it finally transmits its data on the RACH channel. The RNC returns a Cell Update Confirm message 14, and the UE sends a UTRAN Mobility Information Confirm message 15. The RNC then sends a Radio Link Setup message 16 to the Node B 17 serving the UE. The Node B returns a Radio Link Setup Response message 18, and the RNC sends a Radio Bearer Reconfiguration message 19 to the UE. The Node B sends a Radio Link Restore Indication 21 to the RNC, and the UE sends a Radio Bearer Reconfiguration Complete message 22 to the RNC. Finally, with the radio bearer obtained, the UE sends a SIP INVITE message 23 to the CSCF 24 to initiate the PTT session.
Steps 14-22 require approximately 800-900 ms in addition to the 200-250 ms for sending the initial Cell Update message 12. Therefore, the procedure illustrated in FIG. 1 requires a total of approximately 1.1-1.2 seconds to send out the SIP INVITE message from the time the end user presses the PTT button. Note that if the UE starts in IDLE state, it will take even longer (over 2 seconds) to send out the SIP INVITE message because a Radio Access Bearer (RAB) must also be set up towards the Core Network.